1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus incorporating the same, and more particularly, to a fixing device that fixes a toner image in place on a recording medium with heat and pressure, and an electrophotographic image forming apparatus incorporating such a fixing device.
2. Discussion of the Background
In electrophotographic image forming apparatus, such as photocopiers, facsimiles, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by attracting toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the recording medium is forwarded to a post-transfer media conveyance path and undergoes a fixing process, which permanently fixes the toner image in place on the recording medium by melting and settling toner with heat and pressure.
Various types of fixing devices are known in the art, most of which employ a pair of parallel, elongated fixing members, at least one of which is heated and/or pressed against the other to define a line of contact called a fixing nip through which the recording medium is ultimately passed to fix the toner image in place. Such fixing members include paired cylindrical rollers and belts, typically equipped with a motor to impart rotational force to drive one of the pair, which in turn rotates the other as it drives a recording sheet through the fixing nip for forwarding along the media conveyance path.
What is required for good performance of a fixing device is to have a reliable and efficient conveyance system for conveying recording sheets upstream and downstream along the media conveyance path. Reliably conveying a recording sheet through the fixing process is difficult, since variations in processing speed through the fixing nip can cause the recording sheet to deviate from the proper path and crease or jam at the fixing nip due to poor coordination with other processes, or interfere with adjacent surfaces to distort or degrade the toner image on its printed face. Moreover, maintaining proper processing speed can put excessive loads on the drive motor, resulting in reduced power efficiency in the fixing device.
The requirement for reliable sheet conveyance is challenging, particularly for modern image forming apparatuses which use a wide range of recording sheets different in thickness, weight, roughness, and surface coating. These high-performance printers employ a fixing device that can accommodate variations in sheet type by adjusting rotation rate of the drive motor, or as in the case for high-speed printers, by adjusting width of the fixing nip along the conveyance path. Unfortunately, such changes in operating condition lead to variations in conveyance speed at the fixing nip, leading to print defects or overloading of the drive motor.
Another factor affecting sheet conveyance in the fixing process is occasional variations in conveyance speed caused by thermal deformation of the fixing members defining the fixing nip. It has been known that a fixing member or roller made of elastic rubber, which is most commonly employed, expands and contracts when heated and cooled during operation. This expansion and contraction varies the speed at which the outer circumference of the fixing member moves as the drive motor rotates at a regulated rotation rate, resulting in the conveyance speed deviating from the proper speed as specified based on the original size of the fixing member.
One approach to overcoming such problems is to alter the rotation rate of the driver motor depending on the temperature at which the fixing member is operated, so as to maintain a constant conveyance speed regardless of thermal deformation of the fixing member. However, this method has a limitation in that it cannot accommodate changes in operating condition where both of the paired fixing members exhibit different degrees of thermal deformation, resulting in print defects caused by inconsistency in conveyance speed at the fixing nip.
Sheet conveyance through the fixing process is even more complicated where the fixing device is employed in combination with a secondary fixing device that provides a glossy and smooth appearance to the fixed image after fixing, typically used for printing on coated paper sheets with high gloss and smoothness on their surfaces.
Conventionally, high gloss printing in electrophotographic printers is performed by a single fixing device that intensely heats an unfixed toner image by slowing the conveyance speed down to below half an original speed and increasing the amounts of heat and pressure applied to thoroughly fuse toner particles into a substantially uniform semi-solid mass. When cooled and fixed, the resulting image exhibits higher gloss than is obtained through the normal fixing process. Such conventional method requires extremely high nip pressure and relatively large equipment for the fixing process to obtain sufficient glossing performance while maintaining productivity at an acceptable level.
Using multiple fixing modules for glossing prints in a single printer can alleviate problems encountered by the conventional approach. That is, a multi-pass fixing method uses a relatively low pressure for each fixing module, which can therefore operate at a relatively high speed, leading to higher productivity and efficiency of glossing compared to the conventional single-pass method.
One example of such multi-pass fixing system uses a primary fixing station in combination with one or more secondary fixing stations. Printing is performed using two different modes of operation, gloss and non-gloss, wherein a recording sheet passes through only the primary fixing station in the non-gloss mode but through both primary and secondary fixing stations in the gloss mode. Such a method has a drawback in that it requires multiple conveyance paths, one for the non-gloss mode and another for the gloss-mode, which is inefficient in terms of space and complexity involved in using two conveyance paths in the single fixing system.
Another method also provides a dual-mode fixing system using primary and secondary fixing stations defining fixing nips arranged in series along a single conveyance path, at least one of which can adjust nip pressure by moving a pair of fixing members relative to each other. According to this method, the fixing stations relay a recording sheet from one to the other along a guide plate therebetween along the conveyance path in both modes of operation, and switches between operation modes by decreasing and increasing the adjustable nip pressure to obtain the desired levels of gloss.
Still another method also proposes a dual-mode fixing system using a series of primary and secondary fixing stations along a single conveyance path, but which adjusts a processing temperature of the secondary fixing station so as to keep this temperature lower than that of the primary fixing station in order to reduce power consumed in the glossing process. This method requires the primary and secondary fixing stations to be sufficiently close to each other to prevent heat from dissipating from the recording sheet in process, and only allows a limited distance between the fixing stations for disposing a guide plate to ensure good stripping of recording sheets from the primary fixing roller even in an oil-less configuration.
A common problem encountered by the conventional methods using a series of primary and secondary fixing stations along a single conveyance path is the difficulty in properly forwarding a recording sheet from the primary to secondary fixing station along the conveyance path through which the recording sheet travels with a toner image immediately after primary fixing, which is still hot and thus exhibits adhesion to adjacent surfaces such as paper guides and conveying rollers, resulting in sheet jamming as well as image distortion. This problem is more pronounced with the primary and secondary fixing stations disposed close to each other, where any inconsistency in conveyance speed between the fixing stations can crease a recording sheet being processed, with one end pinched by the secondary fixing nip and the other end by the primary fixing nip.